Neighbors furnace, 1999 Rheem gas pack and based on my non professional opinion, looks to be in pretty good condition for an older unit.
Neighbor is a senior with limited income and from what I understand she called the HVAC repairman that had replaced the heat exchanger last season for help.
He came out, told her she needed to replace the entire system, took her money and left without repairing the unit. I do hope there is a special place in Hell for people that take advantage of seniors.
I see the cost of a furnace repair in my future so she doesn't freeze this winter, which will have to be my course of action this week as the temperatures are cooling down.
I have a background in electronics so troubleshooting electrical equipment isn't foreign, but I am not an HVAC technician.
Fortunately the owners manual has an excellent troubleshooting guide and schematics.
Furnace goes through the start up sequence, starts running then the Limit Control switch in the air plenum opens and the furnace shuts down.
Once the Limit Control switch cools down and closes the furnace will go through the cycle again.
Plenum seems to get real hot, yet the squirrel cage blower is running. I temporarily by passed the LC switch to see if it could be anything else and the unit kept running, so the issue is either the switch or an air flow issue.
I have ordered a new Limit Control switch, but in the back of my mind can't help but wonder if the blower is actually moving enough air. Especially after seeing the specs on the LC switch at 120 degree open and 110 close.
She said she put in a new filter, which I am going to look at to make sure and insure all the vents are open.
For this issue, you have the Limit Control Switch, motor start capacitor, and the blower motor. Of course everything controlled by the electronic control board.
Going to check voltage to the motor to see if it is at the correct level.
How do I know or can I tell if the blower motor is running at the required speed to keep the LC switch from opening?
I know the blower will spin faster in AC mode but I guess I need to see if I can speed the motor up in Heating mode to see if the LC switch stays closed.
What could cause the blower motor to run at a slower speed, enough to cause the plenum to over heat?
I have a Trance furnace in my house and keep the blower running most of the time. You can obviously hear it if you go into the crawl space.
But the blower on the neighbors system, seems quiet in comparison. Just can't get the possibility of an air flow issue out of my mind.
Any help, thoughts, or suggestions are welcome.